The present invention relates to a transfer material transport device for forwarding a transfer material (sheet) toward an image transfer section of an image carrying body. More particularly, the invention is directed to a transfer material transport device that causes the transfer material to form a loop at a position immediately before the image transfer section.
In image forming apparatuses such as a copier and a printer, a recorded sheet is produced by forming a toner image on an image carrying body such as a photoreceptor drum by an electrophotographic process, transferring the toner image onto a transfer material such as a sheet in the image transfer section by discharge from a transfer corotron or the like, and then fusing the toner image by a fusing unit.
By the way, in image forming apparatuses such as a printer that do not involve an original document, the exposing timing is adjusted to the timing at which a transfer material reaches the photoreceptor drum. If the speed of the transport of the transfer material by a transport roller toward the photoreceptor drum is lower than the circumferential speed of the photoreceptor drum, large tension is exerted on the transfer material, thereby causing an undesirable misregistration in a transferred image. In particular, if the transfer material is thin paper such as tracing paper, the transfer material itself may be damaged. To solve this problem, in the conventional image forming apparatuses, the transport speed of the transport roller is set at a speed slightly higher than the circumferential speed of the photoreceptor drum.
However, where the transport speed of the transport roller becomes higher than the circumferential speed of the photoreceptor drum, the transfer material may form a loop in a guide member that is disposed immediately before the image transfer section. This increases the incidence of skewing of the transfer material. In particular, image deletions and wrinkles are likely to occur in the case where the transfer material is long. To prevent the occurrence of image deletions, there has been proposed a method of bending the lower portion of the guide member located immediately before the image transfer section. However, this is not a viable solution because in some cases the transfer material does not completely go along the guide member.
Japanese Patent Application Unexamined Publication No. Hei. 3-186880 ("Transfer Roller Device") discloses a method of giving tension to a transfer sheet by temporarily stopping the operation of a pair of transport rollers at the instant when the paper engages with a transfer pressure contact portion, to thereby prevent transfer defects.
However, in this method, tension (back tension) is suddenly exerted on the transfer material upon the suspension of the operation of the transport rollers, which causes the transfer material to slip with respect to the photoreceptor drum, to generate a misregistration in an image.